They are, or at least were, plugged with paper towels. This last session may have broken up the plug so they could flush away.

: My house is two years old...the kids crammed paper towels down the toilet...rented snake, snaked both baths...one toilet and one tub...problem solved: two days later....water in all sinks, tubs, showers..: rented snake again....some paper towels came up on end of snake..felt as if pushed through something...now, tonight...one kid filling up tub, I flushed in opposite toilet, and whammmmm! It backed up again....you could hear water trickle through the pipes in garage....plunged and plunged in one bathroom toilet...toilet in opposite bathroom made gurgling noises, toilet gave up, and water went down?????